1. Field of the Invention
The present invention relates to systems and methods for controlling temperatures of fluids in powerplants, and more particularly, to thermal management systems and methods for controlling fuel and lubricant temperatures in gas turbine engines.
2. Description of Related Art
Gas turbine engines, particularly aircraft powerplants, conventionally use engine fuel as a heat sink to cool electric and electronic components such as engine control systems and/or to cool liquids within the engine such as the engine lubricant and hydraulic fluids for engine control and actuation systems. U.S. Pat. No. 2,865,580, No. 3,080,716 and No. 3,779,007 describe examples of these types of cooling systems. Aircraft also conventionally use engine fuel as a heat sink to cool aircraft components like avionics and/or to cool liquids within the aircraft such as gearbox or generator lubricants and hydraulic fluids for aircraft control and actuation systems. U.S. Pat. No. 4,776,536 and No. 4,505,124 describe examples of these types of cooling systems.
One of the drawbacks of this type of engine thermal management is that the heat added to the fuel by the aircraft and/or engine can cause the engine fuel temperature to exceed operational limits. In some cases, it could even fail to provide adequate cooling of the lubricants and hydraulic fluids for the engine or aircraft, causing them to exceed their operational limits as well. Excessive temperatures of the fuel or lubricants and hydraulic fluids can cause the formation of varnish or coke deposits that can foul engine combustor fuel nozzles, oil jets, and other fuel, control, and lubrication systems components. Accordingly, such deposits can adversely affect the fuel delivery and combustion process, as well as the hydraulic controls and the lubrication and cooling of engine bearings and other parts.
Heat exchangers have been used in a variety of ways to maintain both the fuel and engine oil temperature within acceptable limits, including air/oil heat exchangers, fuel/oil heat exchangers, or both. See, for example, U.S. Pat. No. 4,546,605 and No. 4,696,156. Fuel temperature has also been controlled by re-circulating the fuel through the fuel system and back to the fuel tanks at operational conditions where engine fuel consumption alone does not provide sufficient fuel flow to provide adequate cooling. These types of systems sometimes also use heat exchangers to aid in cooling the fuel. See, for example, U.S. Pat. No. 4,020,632 and No. 4,776,536.
FIGS. 1 to 3 are schematic depictions of exemplary aircraft engine thermal management systems employing different combinations of heat exchangers to maintain appropriate temperatures for both the engine fuel and lubricants used in the engine, in the manner discussed above. FIG. 1 illustrates a thermal management system TM1 installed in a ducted fan gas turbine engine of the type shown in U.S. Pat. No. 4,020,632. The system includes an air/oil heat exchanger AOH in the engine's fan bypass duct (see fan bypass duct 26 in U.S. Pat. No. 4,020,632). The heat exchangers referred to in this description are constructed to transfer heat between fluids in two heat exchange paths through the heat exchanger, in a manner well known to aircraft engine designers. A small percentage of the fan bypass duct air flow BPA, flowing in the direction of the arrows, passes through the heat exchanger AOH along a first heat exchange path represented schematically by dashed lines AOP1. Engine oil flowing through an engine oil line EOL in the direction of the arrows passes through another heat exchange path AOP2 of the heat exchanger AOH. The engine lubrication system typically includes multiple sumps similar to the sump 82 in U.S. Pat. No. 4,020,632, from which the oil is introduced to the engine oil line EOL of the thermal management system TM1. After exiting the heat exchanger AOH, the oil is re-introduced to the engine lubrication system through the engine oil line EOL.
The thermal management system TM1 also includes an oil/fuel heat exchanger OFH that transfers heat to the engine fuel from the hot engine oil entering the thermal management system from the engine sumps. A fuel pump EFP pumps fuel from a fuel tank (not shown) through a fuel line in the direction of the arrows in the figure. The engine fuel passes through the heat exchanger OFH along a first heat exchange path represented by solid lines OFP1 and is introduced to the engine from the thermal management system TM1 by an engine fuel line EFL, as regulated by a fuel control valve FCV that receives the fuel from a fuel control line FCL exiting the heat exchanger OFH. The engine oil line EOL passes through a second heat exchange path represented by the dashed lines OFP2 of the heat exchanger OFH before it is introduced into the air/oil heat exchanger AOH.
In operation, the temperature of the hot oil from the engine is reduced by transferring some of its heat content to the fuel being pumped to the engine through the heat exchanger OFH. By the same token, the temperature of the fuel is increased by the heat thus extracted from the oil. As noted above, the fuel temperature cannot exceed certain limits, so the amount of heat that can be transferred to the fuel from the hot engine oil must be maintained at levels that will not cause the fuel to overheat. Other variables to be taken into account are the different fuel flow rates and heat load on the engine oil at different aircraft flight regimes. As a result, the operating characteristics of the oil/fuel heat exchanger OFH may not reduce the oil temperature sufficiently for re-introduction to the engine. As a result, a second heat exchanger, the air/oil heat exchanger AOH described above, is incorporated into the system to further manage the engine oil temperature. This not only adds weight to the aircraft, but also creates a pressure loss in the fan bypass duct airflow BPA, resulting in a reduction in propulsive thrust.
FIG. 2 schematically depicts another example of a typical thermal management system TM2 that can be used on an aircraft engine such as that described in U.S. Pat. No. 4,020,632. For ease of understanding, similar references are used in FIG. 2 to denote system components that generally correspond to like components in the system depicted in FIG. 1. Accordingly, not all of the references in FIG. 2 are mentioned in this description. As in FIG. 1, the arrows on the depicted fluid flow paths indicate the direction of fluid flow.
Referring to FIG. 2, the system TM2 includes an oil/fuel heat exchanger OFH that transfers heat from the hot engine oil introduced to the system from the engine, in a fashion similar to that employed in the oil/fuel heat exchanger OFH described in connection with FIG. 1. In contrast to the system TM1 in FIG. 1, the oil returns directly to the engine from the heat exchanger OFH in FIG. 2. In addition, the system TM2 includes an air/fuel heat exchanger AFH that transfers heat from the fuel to a fraction of the aircraft inlet or nacelle airflow NCA. The fuel exiting the heat exchanger OFH passes through a recirculation valve RCV that determines the amount of fuel flow through a fuel recirculation line FRL back to the fuel tank (not shown) after cooling by virtue of flowing through the heat exchanger AFH. This re-circulated fuel flow increases the fuel flow through the oil/fuel heat exchanger OFH beyond that provided by fuel control valve FCV to the engine combustors (not shown) through the fuel control line FCL. This increase in fuel flow through the oil/fuel heat exchanger OFH results in an increase in the heat transferred from the engine oil to the fuel and also results in a reduction in fuel temperature entering and exiting the heat exchanger OFH.
This type of system is more effective in using the fuel as a heat sink for the engine oil. It enables the fuel to be maintained at a sufficiently low temperature to adequately cool the engine oil under more varied engine operating conditions, because it does not involve the design compromises that must be built into systems like that shown in FIG. 1, the fuel flow rates of which are limited to that required for engine operation. For example, under engine operating conditions in which the heat load on the engine oil is high, and/or the fuel flow rate required for engine operation is low, the system in FIG. 2 permits more fuel to be re-circulated through the heat exchanger AFH by increasing the fuel flow rate delivered by the pump EFP. By employing appropriate heat-sensing instrumentation, such as the temperature control device TCD illustrated schematically in the engine fuel control line FCL, a servo system can modulate the flow through the valve RCV to maintain a desired fuel temperature limit. In this manner, a thermal management system along the lines of TM2 can cope with a wide variety of engine operating conditions. However, the system may still not be capable of handling all engine and aircraft operating conditions without exceeding either the fuel or engine oil operating temperatures limits, resulting in operational limits on the aircraft.
FIG. 3 shows yet another typical thermal management system TM3, which is particularly adapted for gas turbine engines with separate gearing structures (“gearboxes”) for certain engine components. Many existing turbofan aircraft engines have plural engine shafts that rotate at different speeds to better match the rotational speeds of the fan and compressor components to the turbines that are used to drive them. Turboprop engines typically have gearboxes through which the engine main shaft is connected to the propeller to better match the rotational speed of the propeller to the turbine that drives it. Moreover, in the search for greater fuel efficiency, future aircraft gas turbine powerplants are expected to employ geared turbofans to reduce fan speed and engine noise and to incorporate “open rotor” designs to the same ends. U.S. Pat. No. 7,309,210 illustrates an exemplary geared turbofan design, and U.S. Pat. No. 4,171,183 shows a typical “open rotor” design. Gearboxes for these types of engines may have their own lubricating system separate from the main engine lubricating system, as illustrated by the turboprop system disclosed in U.S. Pat. No. 4,999,994. Thus, an engine with a separate gearbox creates another heat load that the engine's thermal management system must account for.
Referring to FIG. 3, the thermal management system TM3 includes separate heat exchangers for the main engine lubricating system and the gear box lubricating system. As in FIG. 2, similar references are used in FIG. 3 to denote system components that generally correspond to like components in the systems depicted in FIGS. 1 and 2, so that not all of the references in FIG. 3 are mentioned in this description. As in FIGS. 1 and 2, the arrows on the depicted fluid flow paths indicate the direction of fluid flow.
The thermal management system TM3 includes an air/oil heat exchanger AOH in a gearbox lubricating oil line GOL. In a fashion similar to that employed with the engine lubricating oil, the gearbox oil re-circulates from a sump (not shown), through the heat exchanger AOH, and then back to the gear box (not shown). A separate heat transfer line EOL for the engine lubricating oil passes through an oil/fuel heat exchanger OFH, so that heat from the engine lubricating oil is transferred to the fuel, in a fashion similar to that described in connection with the heat exchangers OFH in FIGS. 1 and 2.
Features from the thermal management systems described in FIGS. 1 to 3 may also be combined in nearly limitless combinations and permutations. For example, a fuel re-circulation line and its associated heat exchanger in the system TM2 depicted in FIG. 2 could be incorporated into the systems depicted in FIGS. 1 and 3. Some combinations are shown in U.S. Pat. No. 2,865,580, No. 3,080,716, No. 3,779,007, No. 4,776,536, and No. 4,999,994. It should also be noted that the relatively hot engine oil collected from the multiple sumps within the lubrication system may be combined into a single engine oil line (EOL) or gearbox oil line (GOL) as depicted in FIGS. 1-3. Alternatively, these lines may remain separate, for introduction into the heat exchangers via multiple heat exchange paths. Moreover, when multiple lines are used, lubricant from some sumps may not be introduced to more than one heat exchanger, as distinguished from the system depicted in FIG. 1 for example. In summary, FIGS. 1-3 are meant to be representative examples of conventional thermal management systems, and there have been myriad variations on basic systems such as those described above. In systems with multiple oil sumps, the exact configuration can depend on the operating temperature ranges of the oil in the sumps.
Another avenue used to address the management of engine fuel temperatures is to minimize the effects on the engine fuel of excessive temperature by increasing the temperature limits that the fuel can tolerate. In that regard, U.S. Pat. No. 6,939,392 suggests deoxygenating the fuel to allow the fuel to get hotter before the onset of coking, and U.S. Pat. No. 5,264,244 discloses coating fuel system components to reduce coke deposition. However, it would still be desirable to be able to use the fuel to extract heat from the engine or gearbox oil as described above. Permitting the engine fuel to operate at even higher temperature levels can make it more difficult to use the fuel in that capacity.
Although there are ways to tailor prior thermal management systems to the operational requirements of particular engines, and to raise the temperature limits that can be tolerated by the fuel, prior approaches rely heavily on the basic technology of using heat exchangers in different combinations in an effort to provide the required degree of temperature control. In addition, as these systems get more complex, they produce a greater weight penalty, and most important, the limitations inherent in their designs do not provide the maximum degree of temperature control.
There are examples of attempts to make gas turbine engine thermal management systems transfer heat across unfavorable temperature gradients using heat pumps. For example, U.S. Pat. No. 6,182,435 uses a “compression/expansion cooling device” to transfer heat from lower temperature fuel to higher temperature fan duct air. See also U.S. Pat. No. 6,939,392 (heat pump 100). However, no known prior art incorporates a heat pump into an engine thermal management system in a way that takes advantage of the ability of a heat pump to change the direction in which heat is transferred.